Breakup Patterns Research Articles

Numerical study of droplet breakup and merging in a microfluidic channel

Droplet breakup and merging in a microfluidic channel, which are applied to lab-on-a-chip devices for biomedical testing and synthesis, are simulated numerically by solving the conservation equations of mass and momentum. The droplet surface is computed using the volume-of-fluid method of the commercial code FLUENT. The numerical simulation demonstrates that the variation of obstacle geometry in a microchannel determines the droplet breakup pattern and the volume fraction of split droplets. The computation also shows that droplet merging depends on the channel-chamber width ratio. The effect of microchannel and obstacle configuration on the droplet motion is investigated to find the optimal conditions for droplet breakup and merging.

Electron-electron collision dynamics of the four-electron escape in Be close to threshold

We explore the escape geometry of four electrons a few eV above threshold following single-photon absorption from the ground state of Be. We find that the four electrons leave the atom on the vertices of a pyramid instead of a previously-predicted tetrahedron. To illustrate the physical mechanisms of quadruple ionization we use a momentum transferring attosecond collision scheme which we show to be in accord with the pyramid break-up pattern.

Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics

Understanding how seasonal environmental conditions affect the timing and distribution of synchronized animal movement patterns is a central issue in animal ecology. Migration, a behavioural adaptation to seasonal environmental fluctuations, is a fundamental part of the life history of numerous species. However, global climate change can alter the spatiotemporal distribution of resources and thus affect the seasonal movement patterns of migratory animals. We examined sea ice dynamics relative to migration patterns and seasonal geographical fidelity of an Arctic marine predator, the polar bear (Ursus maritimus). Polar bear movement patterns were quantified using satellite-linked telemetry data collected from collars deployed between 1991-1997 and 2004-2009. We showed that specific sea ice characteristics can predict the timing of seasonal polar bear migration on and off terrestrial refugia. In addition, fidelity to specific onshore regions during the ice-free period was predicted by the spatial pattern of sea ice break-up but not by the timing of break-up. The timing of migration showed a trend towards earlier arrival of polar bears on shore and later departure from land, which has been driven by climate-induced declines in the availability of sea ice. Changes to the timing of migration have resulted in polar bears spending progressively longer periods of time on land without access to sea ice and their marine mammal prey. The links between increased atmospheric temperatures, sea ice dynamics, and the migratory behaviour of an ice-dependent species emphasizes the importance of quantifying and monitoring relationships between migratory wildlife and environmental cues that may be altered by climate change.

Evidence for aT-Shape Break-Up Pattern in the Triple Photoionization of Li

We examine the angular distributions of all three electrons ionized from Li by a single photon near the triple ionization threshold using a fully quantum-mechanical treatment. We find strong evidence for a T-shape break-up pattern at a 5 eV excess energy as previously predicted by quasiclassical simulations [A. Emmanouilidou and J. M. Rost, J. Phys. B 39, 4037 (2006); A. Emmanouilidou, P. Wang, and J. M. Rost, Phys. Rev. Lett. 100, 063002 (2008)]. This finding is in conflict with the expected Wannier break-up dynamics of three electrons moving at mutual angles of 120°, which is expected to hold at energies a few eV above threshold. We use our quantum-mechanical approach to explore the physical mechanisms behind this unusual break-up configuration.

Experimental study of two types of stripping breakup of the drop in the flow behind the shock wave

Results of an experimental study of two regimes of stripping breakup of low-viscosity liquid drops in the flow behind the shock wave in the range of Weber numbers from 200 to 8000 are reported. A phenomenological pattern of drop breakup is constructed on the basis of data on drop deformation and on the flow around the drop. A physical criterion of the change in stripping mechanisms of drop breakup is formulated.

Breakup and vaporization of droplets under locally supersonic conditions

The disruption and vaporization of simulated fuel droplets in an accelerating supersonic flow was examined experimentally in a draw-down supersonic wind tunnel. The droplets achieved supersonic velocities relative to the surrounding air to give relative Mach numbers of up to 1.8 and Weber numbers of up to 300. Mono-disperse, 100 μm-diameter fluid droplets were generated using a droplet-on-demand generator upstream of the tunnel entrance. Direct close-up single- and multiple-exposure imaging was used to examine the features of droplet breakup and to determine the droplet velocities. Laser-induced fluorescence (LIF) imaging of the disrupting droplets was performed using acetone fluorescence to determine the dispersion of the expelled vapor. Three test liquids were employed: 2-propanol and tetraethylene glycol dimethyl ether as non-volatile fluids and a 50/50 hexanol-pentane mixture (Hex-Pen 50/50). The vapor pressure of the Hex-Pen 50/50 was sufficiently high to cause the droplet fluid to potentially become superheated in the decreased static pressure of the supersonic stream. The dynamics for 2-propanol and Hex-Pen 50/50 droplets were similar up to the point of disruption, which occurred more rapidly for the more volatile Hex-Pen 50/50. A 1D dynamic droplet model was developed to provide a first estimate of the expected droplet acceleration and velocity. The actual droplet velocities were in reasonable agreement with the model up to the point at which significant droplet disruption and mass loss commenced. The droplet deformation and breakup patterns for these supersonic flow conditions can be classified into four different flow regions characterized by changes in the Weber number with downstream distance as the droplets accelerate, however, those flow regimes and Weber number ranges were different than those seen for droplets disrupting in shock tubes. The disruption patterns were seen to be generally similar for the different fluids, though droplet disruption occurred more rapidly for the more volatile fluid. LIF imaging established the extent of the dispersion of the expelled vapor. Examination of the vapor clouds surrounding the droplets suggests that Hex-Pen 50/50 droplets had a greater rate of vaporization than 2-propanol droplets starting at approximately 2 mm downstream of the nozzle throat, where the air static pressure became lower than the liquid vapor pressure. This suggests that droplet superheating can have an effect on the extent and rate of droplet vaporization under locally supersonic conditions. The degree of vaporization for Hex-Pen 50/50 was approximately 1.3 times greater than that of the non-volatile fluids over all downstream distances in the supersonic flow.

Coastal marsh die-off and reduced attenuation of coastal floods: A model analysis

Global climate change is expected to increase the risks of coastal flood disasters due to accelerating sea level rise and increasing intensity and frequency of storm surges. Coastal marsh vegetation is considered, on the one hand, to increase resistance to a landward propagating flood wave, such as a storm surge, and hence to protect against flood disasters. On the other hand, coastal marsh vegetation is dying off at several places around the world due to accelerating sea level rise. Here we present hydrodynamic model simulations of flood attenuation by a tidal marsh, with particular focus on the effects of spatial patterns of vegetation die-off. It is shown that a same percentage of marsh die-off but occurring as different spatial patterns of marsh break-up has largely different effects on flood attenuation. Patches of die-off that are directly connected to tidal channels have a much greater effect on increased landward flood propagation, while a same percentage of marsh die-off, but occurring at inner marsh locations disconnected from tidal channels, has only a minor effect. This implies that a random pattern of up to 50% of marsh die-off still provides a considerable flood attenuating effect. However with increasing percentage of random marsh die-off the flood attenuating effect decreases exponentially, since the chance for vegetation die-off occurring directly adjacent to tidal channels increases. This study demonstrates that tidal marsh die-off, which may increase with ongoing global change, is expected to have non-linear effects on reduced coastal protection against flood waves.

207 気流による液泡分裂に関する液体物性の効果(流体工学(乱流・混合・拡散))

To reduce the greenhouse gases, bio fuels are developing as an alternative fuel instead of fossil fuels. As carbon dioxide and so on are the causes of global warming, low carbon combustion is one of the crucial issues to conserve the global environment. Liquid atomization technology should contribute to the field of fuel combustion and others, such as the cooling of exhausted warm water at geothermal power plants and so on. A liquid bubble contains gas in a liquid drop. So, it has a larger surface area than a liquid drop of the same mass. It could have the advantage for fuel combustion and cooling of exhausted warm water. In order to apply it in these areas, effects of liquid physical properties for liquid bubble breakup due to airstreams were investigated. Experiments were conducted using a horizontal air-suction-type wind tunnel. Liquid and air were fed to a nozzle, and uniformly sized liquid bubbles were produced. The liquids used in the experiment were water and several kinds of ethanol in water solutions. The breakup processes and characteristic breakup patterns were precisely observed using a digital high-speed video camera. The displacement of liquid bubbles in the airstream was measured in the entire process. The deformation rate was also measured. The author has been studying on the deformation and breakup of liquid bubbles and liquid drop, such as breakup [1-3], displacement [4, 5], deformation [6, 7]. The liquid bubble has many important scientific applications such as the cooling of warm exhausted water, fuel combustion [8], and a compound liquid jet [9]. It is also useful for the study of structure of collapsing liquid drops and liquid bubbles by shock waves. By comparing a structure of collapsing liquid bubbles with liquid drops, the collapsing mechanism was revealed in a high-speed airstream behind a shock wave [10-12]. If liquid bubbles are used to combust bio-ethanol fuel, biodiesel fuel (BDF), and glycerin which is a by-product of BDF, more effective combustion could be obtained than in the case of liquid droplets. The experiment was conducted to investigate the effects of liquid physical properties for liquid bubbles breakup due to airstreams. The breakup pattern is impacted due to surface tension, and viscosity of liquid. Several characteristic deformation and breakup patterns are observed. Some of them are never observed for liquid drops. The breakup processes and characteristic breakup patterns are precisely observed. The displacement of liquid bubbles in the airstream is measured in the entire process. The deformation rate, that is, a time variation of liquid bubble diameters is also measured. These data are obtained by analyzing the breakup processes recorded by the digital high-speed video camera.

Premixed flame flashback in wall boundary layers studied by long-distance micro-PIV

Premixed flame propagation against the main flow direction in wall boundary layers, a situation known as wall flashback, has been studied by long-distance particle image velocimetry with spatial micron resolution (μ-PIV) and simultaneous flame luminescence recordings. Numerical simulations of laminar wall flashback assist the interpretation of the experimental results. Inside a turbulent boundary layer, the flame propagates in discrete flame cusps pointing in upstream direction and showing a well-defined pattern of formation and break-up. In a laminar boundary layer, the leading flame region is smooth and exhibits low curvature. The instantaneous velocity fields reveal a backflow region upstream of the flame during flashback, which is constrained to the leading flame zone. The backflow is caused by an interaction between the pressure increase upstream of the flame and the boundary layer. The flashback limit is controlled by thermal quenching of the flame tip in the backflow region.

Probabilistic representations of partial branching ratios: bridging the gap between experiments and chemical models

Dissociative recombination (DR) of polyatomic positive ions with electrons is a very complex process, for which the breakup patterns into neutral fragments are presently unpredictable. Measured branching ratios to the various products channels are thus the only data source for modelers of chemical plasmas. Because of limited detector resolution, data for many ions are incomplete, and implementation of DR processes in chemical models is problematic. In a recent paper [Plessis S et al. 2010 J. Chem. Phys. 133 134110], we proposed a novel approach, based on probabilistic trees, enabling modelers to fully account for the hierarchy of available information. For the first time, we were able to build a chemical model for Titan's ionosphere including all available data about DR branching ratios. The improvement on previous models is considerable, and the data needs for Titan are redefined accordingly.

Nanoliter-Droplet Breakup in Confined T-Shaped Junctions

Nanoliter-droplet breakup in either symmetrically or asymmetrically confined T-shaped junctions is experimentally studied. The critical condition with which nanoliter droplets will break equally is theoretically analyzed based on the pressure-driven mechanism. The scaling analysis is experimentally confirmed, implying that the droplet breakup in a confined T-shaped junction is a pressure-driven process when the capillary number is less than ∼0.1. A semi-empirical correlation is obtained for predicting the equal breakup in symmetric T-shaped junctions. The critical condition is found to be dependent on the initial droplet length, channel depth and capillary number. Besides the equal breakup of nanoliter droplets, a new droplet breakup pattern, unequal breakup, is observed in the symmetric T-shaped junction. In asymmetric T-shaped junctions the nanoliter-droplet breakup is found to be very difficult. Keywords: Nanoliter droplet, droplet breakup, confined T-shaped junction, critical condition, microfluidics, capillary number, scaling analysis, interfacial tension, microchannels, flow stability

Origins and Levels of Seasonal Forecast Skill for Sea Ice in Hudson Bay Using Canonical Correlation Analysis

Abstract Canonical correlation analysis (CCA) is used to estimate the levels and sources of seasonal forecast skill for July ice concentration in Hudson Bay over the 1971–2005 period. July is an important transition month in the seasonal cycle of sea ice in Hudson Bay because it is the month when the sea ice clears enough to allow the first passage of ships to the Port of Churchill. Sea surface temperature (quasi global, North Atlantic, and North Pacific), Northern Hemisphere 500-mb geopotential height (z500), sea level pressure (SLP), and regional surface air temperature (SAT) are tested as predictors at 3-, 6-, and 9-month lead times. The model with the highest skill has three predictors—fall North Atlantic SST, fall z500, and fall SAT—and significant tercile forecast skill covering 61% of the Hudson Bay region. The highest skill for a single-predictor model is from fall North Atlantic SST (6-month lead). Fall SST explains 69% of the variance in July ice concentration in Hudson Bay and a possible atmospheric link that accounts for the lagged relationship is presented. CCA diagnostics suggest that changes in the subpolar North Atlantic gyre and the Atlantic multidecadal oscillation (AMO), reflected in sea surface temperature, precedes a deepening/weakening of the winter upper-air ridge northwest of Hudson Bay. Changes in the height of the ridge are reflected in the strength of the winter northwesterly winds over Hudson Bay that have a direct impact on the winter ice thickness distribution; anomalies in winter ice severity are later reflected in the pattern and timing of spring breakup. July ice concentration in Hudson Bay has declined by approximately 20% per decade between 1979 and 2007, and the hypothesized link to the AMO may help explain this significant loss of ice.

Aerobreakup of Newtonian and Viscoelastic Liquids

In this review, we consider and unify all aspects of the dynamics of Newtonian and viscoelastic liquid drops in high-speed gas flows, including shock waves. The path to understanding is opened by novel, laser-induced fluorescence visualizations at spatial resolutions of up to 200 pixels for millimeter and exposure times as low as 5 ns. The central role of the competition between Rayleigh-Taylor and Kelvin-Helmholtz instabilities is assessed in the frame of rich aerodynamics, from low subsonic to supersonic, and the multitude of characteristic length scales and timescales at play with varying liquid properties. Acceleration and liquid redistribution (drop deformation) early in the evolution set the stage for this competition, and we insist on an interpretation of the drag coefficient that is physically meaningful. Two principal breakup regimes (patterns of bodily loss of coherence) are identified depending on whether the gas finds its way through the liquid mass, causing gross disintegration, or goes around to induce, through shear, a surface-layer peeling-and-ejection action. Corresponding criticalities are quantified in terms of key physics, consistent with experiments. This covers in a unified fashion all liquids, independent of viscosity and elasticity, and the potential role of direct numerical simulations in supporting further advances is forecast. The resulting particle-size distributions (in a final equilibrium cloud) depend crucially on the pattern of breakup, although in this respect the role of elasticity obtains a special significance in terms of the underlying entangled-polymer-chain dynamics. From a more general perspective, we explain the canonical significance of this fundamental problem and summarize the wide range of its practical relevance, including the recently renewed interest in predicting shock-induced fluidization (or high-speed, atmospheric dissemination) of large masses of liquid agents (so-called weapons of mass destruction).

Patterns of breakup

Patterns of breakup

Pattern Formation of Hollow Drops from Final Breakup of a Hollow Jet

An experimental investigation on the formation and breakup of a hollow jet issuing from a coaxial nozzle into ambient air has been carried out. The hollow jet consists of an outer jet of water that encloses an inner jet of argon gas. By varying the flow rate ratio of the inner jet to the outer jet, three different breakup patterns of hollow water jets are identified using two types of nozzles: (I) mixed hollow and simple drop formation, (II) single-core hollow drop formation, and (III) multi-core hollow drop formation. These patterns are mapped in a space of Weber number versus flow rate ratio, We-Q. Experimental results in pattern (II) show that increasing the flow rate ratio results in increasing the formation frequency, slightly increasing outer diameters of the hollow drops, and decreasing their wall thickness.

Drop deformation and breakup in polystyrene/high-density polyethylene blends under oscillatory shear flow

Drop deformation and breakup in polystyrene/high-density polyethylene viscoelastic melt blends were investigated under the effects of viscosity ratio, the time scale ratio, and droplet elasticity under oscillatory shear flow using an optical flow cell. The deformation was studied in terms of deformation parameters, Def∗=a∗−c/a∗+c, where a∗ and c are the apparent drop principal axes and the minor axes of the droplets as measured from the time series of images. Amplitudes of deformation parameters are defined as the difference between the maximum and minimum values divided by two. The amplitudes increase linearly at small capillary number and nonlinearly at large capillary number, where the capillary is defined as the ratio between the matrix viscous force and the interfacial tension force. The deformation amplitude parameters decrease with increasing viscosity ratio, time scale ratio, and elasticity at any fixed capillary number. Drop breakup patterns observed are the nonsymmetric one-end tearing pattern for the system with a lower viscosity ratio and the two-end stretching and twisting for the system with a higher viscosity ratio. The critical capillary number increases with viscosity ratio but varies slightly with the time scale ratio.

Experimental study of atomization patterns produced by the oblique collision of two viscoelastic liquid jets

Experimental observations and analysis are presented for the formation and atomization of the fluid sheet created by obliquely colliding jets of viscoelastic fluids. Solutions of mono-disperse polystyrene (PS) in diethylphthalate and of poly-disperse polyethylene oxide (PEO) in glycerol/water mixtures were used to investigate the effects of fluid elasticity on the break-up patterns generated by the impact of two jets ejected from nozzles with an internal diameter of 0.85 mm. Various regimes of behaviour were identified which depend on the jet speed. The structures observed for these elastic fluids differ somewhat from those previously reported for Newtonian viscous fluids, and also show different behaviours depending on the degree of viscoelasticity. This study focuses on the periodic atomization, the so-called fishbone pattern, which occurs when the impinging jets form a liquid sheet which breaks up into a regular succession of ligaments and droplets. High-speed flash photography reveals that low concentrations of polymers significantly affect the evolution of the sheet and its fragmentation, the shapes of the ligaments, and the final drop sizes. The maximum fishbone angle is defined and shown to be a useful tool to describe the variation of the atomization pattern with polymer concentration. For the PS solutions the variation of maximum fishbone angle with reduced polymer concentration ( c/c * ) follows a single master curve, but although the same is true for PEO with high molecular weights, the curves remain separate for low molecular weights. Observation of the fishbone patterns formed by the oblique impact of jets may provide a useful tool to observe and characterize inter-chain interaction in high speed extensional flow of polymer solutions.

Liquid Atomization Using a Rotary Bell Cup Atomizer (Influence of Flow Characteristics of Liquid on Breakup Pattern)

This present study investigates liquid flow patterns on the inner surface of a rotary bell cup atomizer and the effect of these flow patterns on the breakup patterns at the edge of the rotary bell cup. It also estimates the atomization characteristics based on the diameter of liquid ligaments issuing from the cup edge. The cup had an outer diameter of 70 mm and the following experimental conditions were used: rotational speed N = 1,000 to 50,000 rpm and liquid flow rate Q = 50 to 300 mL/min. Classification of the breakup pattern at the cup edge revealed that, irrespective of the flow rate, several liquid ligaments were produced at low rotational speeds and fine ligaments, which cause Rayleigh breakup, were generated at high rotational speeds. Classification of the liquid flow pattern on the cup surface revealed that the liquid film on the surface was a smooth film at low rotational speeds and a radial streak film or a partial dry film at high rotational speeds. Good atomization characteristics are expected since the radial streak film and the partial dry film cause fine ligament breakup at the cup edge. It was also demonstrated that the mean droplet size for Rayleigh breakup, which generates uniform droplets, can be effectively estimated by calculations based on the diameter of the ligaments that form on the cup edge.

Ordered microdroplet formations of thin ferrofluid layer breakups

The ordered breakup pattern of a thin layer of ferrofluid drop subjected to a uniform perpendicular field is experimentally investigated. The results confirm a universal pattern formation containing numerous breaking droplets of a uniform size, which is independent of the initial area of ferrofluid drop and the propagating directions of the formation waves. Two quantitative observations regarding the size and number of breaking droplets are concluded. Both the experiments and theoretical analysis show the correlation between the diameter of breaking droplets (d) and magnetization strength (M) can be characterized as d∝1/M2. The uniform size of breaking droplets under a constant field strength results in a linear proportionality between the number of breaking droplets (N) and the initial area of ferrofluid drop (A) as N∝A, which is verified by the experiments.

Tinamous and Moa Flock Together: Mitochondrial Genome Sequence Analysis Reveals Independent Losses of Flight among Ratites

Ratites are large, flightless birds and include the ostrich, rheas, kiwi, emu, and cassowaries, along with extinct members, such as moa and elephant birds. Previous phylogenetic analyses of complete mitochondrial genome sequences have reinforced the traditional belief that ratites are monophyletic and tinamous are their sister group. However, in these studies ratite monophyly was enforced in the analyses that modeled rate heterogeneity among variable sites. Relaxing this topological constraint results in strong support for the tinamous (which fly) nesting within ratites. Furthermore, upon reducing base compositional bias and partitioning models of sequence evolution among protein codon positions and RNA structures, the tinamou-moa clade grouped with kiwi, emu, and cassowaries to the exclusion of the successively more divergent rheas and ostrich. These relationships are consistent with recent results from a large nuclear data set, whereas our strongly supported finding of a tinamou-moa grouping further resolves palaeognath phylogeny. We infer flight to have been lost among ratites multiple times in temporally close association with the Cretaceous-Tertiary extinction event. This circumvents requirements for transient microcontinents and island chains to explain discordance between ratite phylogeny and patterns of continental breakup. Ostriches may have dispersed to Africa from Eurasia, putting in question the status of ratites as an iconic Gondwanan relict taxon.